The United States military definition in the Department of Defense Dictionary of Military and Associated Terms comes from Joint Publication 3-05.1 – Joint Special Operations Task Force Operations (JP 3-05.1).[19] JP 3-05.1 defines a "special mission unit" as "a generic term to represent a group of operations and support personnel from designated organizations that is task-organized to perform highly classified activities."[20]

The U.S. government does not acknowledge which units specifically are designated as special missions units,[9] only that they have special mission units under U.S. Special Operations Command. However, in the early 1990s then-Commander in Chief of U.S. Special Operations Command, General Carl Stiner identified both Delta Force and SEAL Team Six as permanently assigned special missions units in congressional testimony and public statements.[21] In 1998 Under Secretary of Defense for PolicyWalter B. Slocombe publicly referred to special mission units during a briefing to the Senate Armed Services Committee; "We have designated special-mission units that are specifically manned, equipped and trained to deal with a wide variety of transnational threats" and "These units, assigned to or under the operational control of the U.S. Special Operations Command, are focused primarily on those special operations and supporting functions that combat terrorism and actively counter terrorist use of weapons of mass destruction (WMD). These units are on alert every day of the year and have worked extensively with their interagency counterparts."[8]